Cu/Cu2O interface nanostructures for electrochemical CO2 reduction

ABSTRACT

A method of electrochemical reduction of CO 2  includes the use of a catalyst of Cu/Cu 2 O particles including Cu/Cu 2 O interfaces. The catalyst may be included in an electrochemical cell for the conversion of CO 2  to value-added products. The electrochemical cell may include an anode, a cathode including the Cu/Cu 2 O particles including Cu/Cu 2 O interfaces, and an aqueous medium containing CO 2  or CO 3   −2 . The CO 2  or CO 3   −2  is reduced by contacting the Cu/Cu 2 O particles with the aqueous medium while supplying electricity to the cell. The conversion of CO 2  by the electrochemical reduction thereof has higher Faradaic Efficiency due to the Cu/Cu 2 O interfaces in the Cu/Cu 2 O particles.

BACKGROUND

Global energy consumption is primarily reliant on the burning of fossil fuels, but sustainable alternatives need to be developed to guarantee a long-term economic growth while mitigating environmental problems from increases in anthropogenic emissions of carbon dioxide (CO₂). To address this concern while simultaneously addressing rising concentrations of atmospheric CO₂, there has been proposed the electrochemical reduction of carbon dioxide to carbon-based value-added products, which themselves can be used as fuels or fuel precursors. A cost-effective process for the electrochemical conversion of carbon dioxide to value-added products may require electro-catalysts that are efficient, selective, and stable.

Nobel metals are potentially candidates for electro-catalysts for the conversion of CO₂. Copper is an inexpensive metal, and thus a potential candidate for being an electrocatalyst, and copper nanostructures possess more surfaces and active sites for electrochemical CO₂ reduction.

While cuprous oxide (Cu₂O) is a good photocatalyst, copper (Cu) is a good electrocatalyst. Both of these catalysts are used for the reduction of CO₂. However, although metallic Copper (Cu) is potentially a good electro-catalyst for CO₂ reduction, oxidation of Cu surface impedes the electro-catalytic activity. Therefore, unintentional oxidation of Cu is detrimental to the electrochemical CO₂ reduction, in particular, increasing overpotential of the reduction.

On the other hand, cuprous oxide (Cu₂O) can efficiently couple with hydrogen in the process of electrochemical CO₂ reduction. Cuprous oxide (Cu₂O) is one of the three stable oxide forms of Cu which have an oxidation state of +1. Cu₂O has a lattice constant of 4.2696 Å. Cu atoms are arranged in a face-centered manner while O atoms are arranged in a body-centered manner. Cu atoms are linearly coordinated to two O atoms while O atoms are tetrahedrally coordinated to four Cu atoms. Previous studies were mainly focused on the synthesis of Cu₂O on Cu films. However, studies on the surface areas and active sites on Cu films are lacking.

BRIEF DESCRIPTION

According to one aspect, a method of electrochemical reduction of CO₂ includes providing an electrochemical cell including an anode, and a cathode including Cu/Cu₂O particles with Cu/Cu₂O interfaces. An aqueous medium containing CO₂ or CO₃ ⁻² is introduced into the cell. The CO₂ or CO₃ ⁻² is reduced by contacting the particles with the aqueous medium while supplying electricity to the cell.

According to another aspect, electrochemical cell for electrochemical reduction of CO₂ or CO₃ ⁻² includes an anode; a cathode including Cu/Cu₂O particles having Cu/Cu₂O interfaces; an electrolyte arranged between the anode and the cathode; and an aqueous medium containing CO₂ or CO₃ ⁻² in contact with the cathode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an SEM image of Cu₂O crystals.

FIG. 2 is a size distribution chart for Cu₂O crystals of FIG. 1 .

FIG. 3 is an EDS layered image of Cu₂O crystals of FIG. 1 .

FIG. 4 is an EDS elemental mapping of the Cu₂O crystals of FIG. 1 .

FIG. 5 is an EDS spectrum of the Cu₂O crystals of FIG. 1 .

FIG. 6 is an SEM image of Cu/Cu₂O particles according to the present subject matter.

FIG. 7 is an SEM close up image of the Cu/Cu₂O particles of FIG. 6 .

FIG. 8 is an SEM far away image of the Cu/Cu₂O particles of FIG. 6 .

FIG. 9 is an EDS spectrum of the Cu/Cu₂O particles of FIG. 6 .

FIG. 10 is an XRD pattern of Cu₂O crystals of FIG. 1 .

FIG. 11 is an XRD pattern of Cu/Cu₂O particles of FIG. 6 .

DETAILED DESCRIPTION

The present subject matter includes copper oxide nanostructures (also referred to herein as crystals) being subject to a controlled reaction so as to only partially reduce the copper oxide (Cu₂O) to elemental copper (Cu), which thereby forms Cu/Cu₂O structures (also referred to as particles) including both elemental copper and copper oxide.

The Cu/Cu₂O particles have high-energy interfaces between the Cu and the Cu₂O, which interfaces are referred to herein as Cu/Cu₂O interfaces. The Cu/Cu₂O interfaces provide active sites for CO₂ electroreduction. The Cu/Cu₂O interfaces minimize their free energy by reconstruction, and enhance molecular adsorption and activation on adsorbate-covered atomically rough interfaces during CO₂ electrochemical reduction. The Cu/Cu₂O particles employ the advantages of both Cu and Cu₂O chemical activity because they include both elemental copper and copper oxide. The Cu/Cu₂O particles can be used in electrochemical CO₂ reduction by way of the high-energy active sites at their Cu/Cu₂O interfaces, which provide a more efficient conversion of CO₂ for producing value-added products such as ethylene glycol, formic acid (HCOOH), methanol (CH₃OH), ethylene (C₂H₄), methane (CH₄), ethane (C₂H₆), or carbon monoxide (CO).

The present subject matter includes a method of forming Cu/Cu₂O particles including Cu/Cu₂O interfaces. The method includes reacting Cu₂O crystals (see for example, FIGS. 1-4 and 10 ) with a reducing agent so as to not entirely reduce the Cu₂O crystals to elemental Cu, and instead only partially reduce the Cu₂O crystals so that some Cu₂O is retained and some is reduced to elemental Cu, thereby forming the Cu/Cu₂O particles having Cu/Cu₂O interfaces (see for example, FIGS. 5-9 and 11 ).

The Cu₂O crystals that are reduced to form Cu/Cu₂O particles, may themselves be synthesized as part of the method, or they may be purchased from a supplier or acquired as naturally occurring crystals.

If synthesized, the Cu₂O crystals may be formed by a wet chemical process under agitation, by preparing a solution including a copper ion contributor dissolved in a solvent, heating the solution to a temperature of from 30-100° C., adding a pH adjuster to the solution so that the solution has a pH of from 2-12, adding a reducing agent to the solution to thereby form a reaction mixture, and reacting the reaction mixture at the temperature of from 30-100° C. for a time of from 1-1000 minutes to thereby precipitate the Cu₂O crystals from the reaction mixture.

The solvent may include any liquid capable of solubilizing the other raw materials, and can include tap or deionized water, aqueous ammonia solutions, or an organic solvent such as methanol, ethanol, acetone, ether, or glycerol for example. In one non-limiting embodiment, the solvent includes deionized water.

The copper ion contributor may be any substance that is capable of contributing copper ions (Cu²⁺), including for example, a copper salt or hydrate thereof. The copper salt can include for example copper (II) chloride (CuCl₂), copper (II) fluoride (CuF₂), copper (II) chloride (CuCl₂), copper (II) bromide (CuBr₂), copper (II) iodide (CuI₂), cuprous iodide (CuI), copper (II) oxide (CuO), copper (II) sulfide (CuS), copper (II) sulfate (CuSO₄), copper (II) nitride (Cu₃N₂), copper(II) nitrate (Cu(NO₃)₂), copper (II) phosphide (Cu₃P₂), copper (II) acetate (Cu(CH₃COO)₂), copper (II) hydroxide (CuOH)₂, copper(II) carbonate (CuCO₃), and copper (II) acetylacetonate (Cu(C₅H₇O₂)₂), or combinations thereof. In a non-limiting example, the copper ion contributor includes copper (II) acetate hydrate (Cu(CH₃COO)₂·H₂O).

The copper ion contributor can be added to the solvent as a solid that is then dissolved therein. The amount of the copper ion contributor used in the reaction is not critical, and the copper ion contributor can be included at an amount to provide a molar concentration in the reaction mixture of 1-40 mM. The copper ion contributor may also be included to provide a molar concentration of 5-15 mM, or 9-11 mM, or 10 mM in the reaction mixture. In one non-limiting example, the copper ion contributor is copper (II) acetate hydrate, and is included in an amount to provide a molar concentration of 35-40 mM in the reaction mixture to synthesize Cu₂O crystals.

The temperature at which the Cu₂O crystals are synthesized may range from 30−100° C., 40-80° C., or 55-65° C.

The pH adjuster may include various acids, bases, or combinations thereof, such as sodium hydroxide (NaOH) or ammonia for example. The pH adjustor may be used to adjust the pH of the reaction mixture to be between 2.0 and 12.0. The pH adjustor may be introduced as a solid for dissolution in the solvent, or as a solution, such as an aqueous solution after the pH adjustor has been dissolved in water. In one non-limiting example, the pH adjustor includes sodium hydroxide, which may be introduced as an aqueous solution to have a molar concentration of sodium hydroxide in the reaction mixture of 10-1000 mM, 100-800 mM, or 500-700 mM. In a non-limiting aspect, a sodium hydroxide aqueous solution is introduced in an amount to provide a molar concentration of 550-650 mM in the reaction mixture.

The reducing agent may include for example, L-ascorbic acid (i.e. vitamin C, or C₆H₈O₆), hydrazine monohydrate, sodium borohydride, hydrazine, 1,2-hexadecanediol, glucose, carbon monoxide, sulfur dioxide, iodides, hydrogen peroxide, oxalic acid, formic acid, carbon, reducing sugars, a borane compound, or combinations thereof.

The reducing agent may be added to the solvent as a solid for dissolution therein, or in a solution, such as an aqueous solution including the reducing agent dissolved in water. In one non-limiting example, the reducing agent is added to the solvent as a solution. The reducing agent may be added in an amount to provide a molar concentration in the reaction mixture of 1-100 millimoles per liter of reaction mixture (i.e. mmol/L or millimolar (mM)), 20-80 mM, or 30-50 mM. In one non-limiting example, the reducing agent includes L-ascorbic acid, which may be introduced as an aqueous solution in an amount to provide a molar concentration of 30-35 mM in the reaction mixture.

The reaction mixture may be reacted for a time of from 1-1000 minutes, 1-100 minutes, or 5-30 minutes.

The Cu₂O crystals are not particularly limited, and may be of any size, include any number of facets, and may have facets of the same type (i.e. same Miller index) or have facets of different types (i.e. different Miller indices). The size of the Cu₂O crystals may range, for example, from 1 nm to 100 μm, or 10 nm to less than 1 μm (i.e. nanosized). The number of facets may range from 4-100. The number of types of facets may range from 1-100. In one non-limiting example, the Cu₂O crystals have an average size of 700-900 nm, have eighteen facets, and have two different types of facets, i.e. twelve (110) facets and six (100 facets). The size of the Cu₂O crystals may determine, as least to some extent, the size of the Cu/Cu₂O particles, which are produced from the Cu₂O crystals. If the size of the Cu₂O crystals are nanosized, then the size of the subsequently produced Cu/Cu₂O particles may also be nanosized.

The Cu₂O crystals may be reacted with a reducing agent to synthesize the Cu/Cu₂O particles having Cu/Cu₂O interfaces. However, in this process the Cu₂O crystals may not be totally reduced to Cu, and instead only partially reduce to thereby form elemental copper and leave some unreacted copper oxide, thus producing the Cu/Cu₂O particles. The molarity of the reducing agent present in the reaction, the duration of the reaction, the temperature at which the reaction takes place, or a combination thereof may be regulated so that the Cu₂O crystals are not fully reduced to elemental Cu. Instead, the Cu₂O crystals are only partially reduced so that the finished product of Cu/Cu₂O nanoparticles includes both elemental Cu and Cu₂O, with interfaces between them.

The synthesis of Cu/Cu₂O may be performed by a wet chemical process using a liquid under agitation. The liquid may include tap or deionized water, aqueous ammonia solutions, or an organic solvent such as methanol, ethanol, acetone, ether, or glycerol for example. In one non-limiting embodiment, the liquid includes deionized water.

The Cu₂O crystals may be reacted with a reducing agent in the liquid for a predetermined time and at a predetermined temperature. The predetermined temperature may range from 40−80° C., 50-70° C., or 55-65° C. The predetermined time may range from 1-60 minutes, 1-30 minutes, 1-10 minutes, or 1-5 minutes.

The reducing agent is included to donate electrons (by oxidation of the reducing agent) that are used for the reduction of the Cu₂O to produce elemental Cu. The reducing agent may include those used to synthesize the Cu₂O crystals disclosed herein. The reducing agent may be added in an amount to provide a molar concentration in the liquid of 10-2000 millimoles per liter of liquid (i.e. mmol/L or millimolar (mM)), 20-1000 mM, or 30-500 mM. In one non-limiting example, the reducing agent includes hydrazine hydrate (N₂H₄·H₂O) at a molar concentration in the liquid of 900-1000 mM.

After partial reduction of the Cu₂O crystals, the resultant Cu/Cu₂O particles may retain the general shape, size, number of facets, and types of facets of the Cu₂O crystals. This is shown between FIG. 1 (showing Cu₂O crystals) and FIG. 6 (showing Cu/Cu₂O particles). However, the facets of the Cu/Cu₂O particles may each present a rough surface (see FIGS. 6-7 ), as opposed to the smooth facets of the Cu₂O crystals (See FIG. 1 ). Such rough surfaces on the facets of the Cu/Cu₂O particles may be due to the reduction of Cu₂O to Cu occurring on these surfaces. The interface between the Cu₂O and the Cu on the surfaces of the Cu/Cu₂O particles may provide actives sites for the electrochemical reduction of CO₂ to value-added products. Thus, the Cu/Cu₂O interfaces present on the surface of the Cu/Cu₂O particles may increase the Faradaic Efficiency of the Cu/Cu₂O particles compared to Cu particles, Cu₂O particles, or combinations thereof of similar amount and similar size.

The present subject matter includes a method of electrochemical reduction of CO₂ using the Cu/Cu₂O particles including Cu/Cu₂O interfaces. CO₂ may be reduced to value-added products such as ethylene glycol, formic acid (HCOOH), methanol (CH₃OH), ethylene (C₂H₄), methane (CH₄), ethane (C₂H₆), ethanol, carbon monoxide (CO), acetic acid, acetone, other organic compounds, or combinations thereof.

Electrochemical reduction of CO₂ may be performed by using the Cu/Cu₂O particles as a catalyst for reducing the CO₂ to value-added products, and such a process may be performed in an electrochemical cell, where CO₂ or CO₃ ⁻² comes into contact with the Cu/Cu₂O particles.

The electrochemical cell used for the reduction of CO₂ may include a cathode having the Cu/Cu₂O particles including Cu/Cu₂O interfaces. The electrochemical cell may further include an anode, an aqueous medium in contact with the cathode, an electrolyte arranged between the anode and the cathode, and other known components of electrochemical cells.

The aqueous medium may include carbon dioxide, which may be included in the aqueous medium by being bubbled through the aqueous medium or being dissolved in the aqueous medium. The aqueous medium may alternatively include carbonate ion (CO₃ ⁻²), which may be formed by dissolving CO₂ in a basic solution, such as aqueous sodium hydroxide solution. The CO₂ or CO₃ ⁻² may be introduced into the cell, and reduced by contacting the Cu/Cu₂O particles with the aqueous medium while supplying electricity to the cell, thereby producing value added products. The Cu/Cu₂O particles may be arranged on a surface of the cathode so as to contact the aqueous medium.

By the electrochemical cell including Cu/Cu₂O particles in the cathode, the Faradaic Efficiency of CO₂ conversion by the electrochemical cell may be increased compared to electrochemical cells including cathodes not having Cu/Cu₂O particles.

Examples

As inventive examples, Cu/Cu₂O particles including Cu/Cu₂O interfaces according to the present invention were produced to evaluate them for use in the electrochemical reduction of CO₂. As a reference, a comparative example was prepared, including crystals of Cu₂O alone.

Comparative Example 1: Cu₂O crystals were synthesized by dissolving 0.6 g of Cu(CH₃COO)₂·H₂O in 60 ml of doubly distilled H₂O in a 250 ml flask under constant electromagnetic stirring. The flask was kept in a 60° C. oil bath. 10 ml of 4.8 M NaOH aqueous solution was added dropwise into the above blue solution and kept stirring for 10 min. Once NaOH was added, a precipitate appeared and the color of the solution gradually changed to dark brown. Then, 10 ml of 0.25 M vitamin C (ascorbic acid) aqueous solution was added into the above suspension and reacted for 20 min, by which a brownish-red product of Cu₂O crystals was produced. The precipitate was separated from solution by centrifugation, washed with ethanol three times and water once, and dried at 50° C. under vacuum overnight.

The structural properties of the synthesized Cu₂O crystals were analyzed by SEM and energy dispersive X-ray. As seen in the SEM images of FIGS. 1 and 3 , the Cu₂O crystals are polyhedral structures with eighteen smooth facets, i.e. six (100) facets and twelve (110) facets. The size distribution of the Cu₂O crystals is shown in FIG. 2 , with an average particle size of about 800 nm±300 nm. FIG. 3 shows an Energy Dispersive Spectroscopy (EDS) layered image, and FIG. 4 shows an EDS elemental mapping of the Cu₂O crystals. FIG. 5 shows an EDS spectrum of the Cu₂O crystals. As can be seen, the atomic ratio of Cu:O in the Cu₂O crystals is 67.3:32.7, or about 2:1.

Example 1: Some of the Cu₂O crystals of Comparative Example 1 were used to form the Cu/Cu₂O particles according to the present invention. In particular, 132 mg of Cu₂O crystals of Comparative Example 1 was added to 40 ml doubly distilled H₂O at room temperature and stirred for 10 min. Then, 2 ml of hydrazine hydrate (N₂H₄·H₂O) was added under magnetic stirring, and heated to 60° C. for 2 min to allow them to react and then passively allowed to cool to ambient temperature. This reaction caused the partial reduction of the Cu₂O crystals to form the Cu/Cu₂O particles. The resulting precipitate of Cu/Cu₂O particles was purified by centrifugation, washed with deionized water three times and dried at 50° C. under vacuum overnight.

The structural properties of the synthesized Cu/Cu₂O particles were analyzed by SEM and energy dispersive spectroscopy. FIGS. 6-8 show SEM images of the Cu/Cu₂O particles, which present facets with a rougher surface than the Cu₂O crystals of Comparative Example 1. The Cu/Cu₂O particles, like the Cu₂O crystals of Comparative Example 1, are 18-facet polyhedral structures, with a representative size of about 788 nm×1.25 μm (FIG. 7 ). The EDS spectrum of FIG. 9 exhibits the characteristic peaks of Cu and O, and the atomic ratio of Cu:O is 94.4:5.6, which indicates that elemental Cu has been produced, and the percentage of copper dominates and oxygen exists. This ratio indicates that there is metallic copper produced in the process, which results in interfaces between the copper and the remaining copper oxide. Such interfaces between the copper and the copper oxide (i.e. Cu/Cu₂O interfaces) will benefit electrocatalytic reduction of CO₂ and other applications.

FIG. 10 shows an X-Ray Diffraction (XRD) pattern of the Cu₂O crystals of Comparative Example 1 before partial reduction to Cu/Cu₂O particles. FIG. 11 shows an X-Ray diffraction patter of the Cu/Cu₂O particles after being formed. It is seen that there are several peaks from Cu in the Cu/Cu₂O particles of Example 1, while there are only peaks for Cu₂O in Comparative Example 1.

Example 1 and Comparative Example 1 were then utilized in the electrochemical reduction of CO₂. One prominent feature of the electrochemical CO₂ reduction by Cu/Cu₂O particles is that ethylene glycol, one of the value-added products for fuel, is generated with a low over-potential. Table 1 below compares Faradaic Efficiency (FE) of CO₂ reduction using Cu/Cu₂O particles of Example 1 (which include Cu/Cu₂O interfaces), with that of Cu₂O crystals of Comparative Example 1 (which do not include Cu/Cu₂O interfaces). All voltages are with respect to Ag/AgCl (pH=6.8).

TABLE 1 Faradaic Efficiency (%) Poten- Formic Ethylene Acetic tial (V) Acid Glycol Acid Ethanol Acetone Example 1 - −1.0 0.72 3.58 0.87 1.03 0.33 Cu/Cu₂O −0.9 0.19 1.95 0.65 0.39 0.12 particles −0.8 0.60 9.22 0.81 3.46 0.45 −0.7 3.1 7.65 1.06 4.46 1.82 −0.6 1.58 15.88 3.56 2.63 0.22 −0.5 1.21 10.7 0.96 3.90 0.78 Comparative −1.4 0.456 4.85 0.21 0.781 0.245 Example 1 - −1.2 0.528 4.89 0.30 0.564 0.151 Cu₂O −1.1 0.46 3.97 0.38 0.413 0.141 crystals −1.0 1.03 12.92 0.495 1.978 0.757 −0.9 0.297 13.16 0.358 1.573 0.358 −0.8 0.959 10.93 0.33 1.24 0.319

It is seen that the FE for ethylene glycol increases from 13.16% for the Cu₂O crystals of Comparative Example 1, to 15.88% with a lower over-potential for the Cu/Cu₂O particles of Example 1. These results may be significant for commercial use because of the low over-potential of CO₂ reduction using Cu/Cu₂O particles with Cu/Cu₂O interfaces, rather than Cu₂O particles alone.

In the present invention, the advantages of both Cu and Cu₂O are exploited for CO₂ reduction. Cu/Cu₂O interfaces minimize their free energy by reconstruction and enhance molecular hydrogen adsorption and activation on CO₂-covered atomically rough interfaces in the CO₂ electrochemical reduction.

It will be appreciated that various of the above-disclosed and other features and functions, or alternatives or varieties thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims. 

The invention claimed is:
 1. A method of electrochemical reduction of CO₂ or CO₃ ⁻² comprising: providing an electrochemical cell including an anode, and a cathode including Cu/Cu₂O particles with Cu/Cu₂O interfaces; introducing an aqueous medium into the cell, the aqueous medium including CO₂ or CO₃ ⁻²; and reducing the CO₂ or CO₃ ⁻² by contacting the Cu/Cu₂O particles with the aqueous medium while supplying electricity to the cell.
 2. The method according to claim 1, wherein the Cu/Cu₂O particles are nanosized.
 3. The method according to claim 1, wherein the Cu/Cu₂O particles include 18-facet polyhedral Cu/Cu₂O particles.
 4. The method according to claim 3, wherein facets of the Cu/Cu₂O particles have rough surfaces and include the Cu/Cu₂O interfaces.
 5. The method according to claim 3, wherein the Cu/Cu₂O particles include twelve (110) facets and six (100) facets.
 6. The method according to claim 1, wherein the Cu/Cu₂O particles have an atomic ratio of Cu:O of more than 2:1.
 7. The method according to claim 6, wherein the Cu/Cu₂O particles have the atomic ratio of Cu:O of more than 10:1.
 8. The method according to claim 1, further comprising preparing the Cu/Cu₂O particles by: providing Cu₂O crystals; and reacting the Cu₂O crystals with a reducing agent at a predetermined temperature for a predetermined time to thereby form the Cu/Cu₂O particles.
 9. The method according to claim 8, wherein: the reducing agent includes hydrazine hydrate; the predetermined temperature is 50-70° C.; and the predetermined time is 1-10 minutes.
 10. The method according to claim 8, wherein the Cu₂O crystals are provided by: forming a solution including a copper ion contributor dissolved in a solvent; heating the solution to a temperature of from 55-65° C.; adding a pH adjuster to the solution, wherein the solution has a pH of from 2-12; adding a reducing agent to the solution to thereby form a reaction mixture; and reacting the reaction mixture at the temperature of from 55-65° C. to thereby precipitate the Cu₂O crystals from the reaction mixture.
 11. The method according to claim 10, wherein the Cu₂O crystals include 18-facet polyhedral particles.
 12. The method according to claim 11, wherein the Cu₂O crystals have an average particle size of 0.8 μm.
 13. The method according to claim 11, wherein the Cu₂O crystals have twelve (110) facets and six (100) facets. 